1. Field of the Invention
The present invention relates to cranes using wire ropes to support loads, and more particularly, to a system for controlling the speed of a motor used to lift the loads.
2. Description of the Prior Art
In tower cranes, truck cranes and other types of cranes, hydraulic motors are used to operate cylindrical drums which reel and unreel wire ropes to hoist and lower loads attached to an outer end of the wire ropes. The hydraulic motors are powered by pressurized hydraulic fluid provided by hydraulic pumps connected to diesel or other engines mounted on the crane. The system may include a fixed displacement motor and a variable displacement pump with the motor and drum speed being determined by the displacement of the pump. A disadvantage of the fixed displacement motor-variable displacement pump is that a large horsepower engine is required to operate the system if high speeds and high torques are to be obtained from the hydraulic motor.
Other cranes use a variable displacement pump and a variable displacement motor to obtain higher operating speeds without using large horsepower engines as long as the load lifted by the crane is fairly light. The variable displacement pump-variable displacement motor combination can also provide relatively high torques at lower speeds if required. However, the hydraulic motor can stall if a crane operator tries to obtain a high speed with a heavy load.